The field of the invention is ultrasonic transducers which radiate ultrasonic waves into the body of a patient and which receive and detect ultrasonic waves emanating from the body of a patient.
Ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements which are sandwiched between a pair of electrodes. Such piezoelectric elements are typically constructed of lead zirconate titanate (PZT), polyvinylidene diflouride (PVDF), or PZT ceramic/polymer composite. The electrodes are connected to a voltage source, and when a voltage is applied, the piezoelectric elements change in size at a frequency corresponding to that of the applied voltage. When a voltage pulse having an ultrasonic frequency is applied, the piezoelectric element emits an ultrasonic wave in the media to which it is coupled. Conversely, when an ultrasonic wave strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. Typically, the front of the element is covered with an acoustic matching layer that improves the coupling with the media in which the ultrasonic waves propagate. In addition, a backing material is disposed to the rear of the piezoelectric element to absorb ultrasonic waves that emerge from the back side of the element so that they do not interfere.
When used for ultrasonic imaging, the transducer has a number of piezoelectric elements arranged in an array and driven with separate voltages (apodizing). By controlling the phase of the applied voltages, the ultrasonic waves produced by the piezoelectric elements combine to produce a net ultrasonic wave which is focused at a selected point. This focal point can be moved in an azimuthal plane to scan the subject. However, objects which are not at this focal plane are out of focus their resolution in the reconstructed image is reduced. Thus, conventional ultrasonic transducers focus the wave providing very high resolution images of objects lying at or near the focal plane, but have increasingly lower resolution of objects lying to either side of this plane. Such transducers are said to have high resolution, but low depth of field.
Nondiffracting solutions to the wave equation have been discovered and extensively tested using electromagnetic waves and ultrasonic waves. Under ideal conditions, these solutions indicate that transducers can produce a wave that is confined to a beam that does not diffract, or spread, over a long distance. Such nondiffractive beams produce a much greater depth of field than other focused beams. In practice, such beams do eventually diffract due to the less-than-ideal transducers and propagating media, and the phrase "limited diffraction beams" has been coined for this class of beams.
In U.S. Pat. No. 5,081,995, an ultrasonic transducer is described which produces one well-known type of limited diffraction beam, which was first discovered by J. Durnin and described in an article entitled "Exact Solutions for Nondiffracting Beams. I. The Scalar Theory", published in the Journal of Optical Society of America, 4(4):651-654 in April, 1987. This limited diffraction beam is referred to as a "Bessel beam" because its lateral beam profile is a Bessel function.
More recently, another type of limited diffraction beam was discovered by J-y. Lu and J. Greenleaf and described in an article "Nondiffracting X waves--exact solutions to free-space scalar wave equation and their finite aperture realizations", IEEE Trans. Ultrason. Ferroelec., Freq. Contr., Vol. 39, pp. 19-31, January 1992. This beam has an x-like shape in a plane along the wave axis and it has been termed "X waves". The X waves are nonspreading in both transverse and axial directions and have a large depth of field even when they are produced by a transducer of finite aperture.
Although limited diffraction beams have a large depth of field, they also have relatively high sidelobes. When viewed along the propagation axis, most of the beam intensity is focused near that axis, but a significant amount of energy is delivered to regions around the focal point. Each type of focussed bean has its own distinctive pattern of these "sidelobes". As shown in FIG. 1 for example, the Bessel beam has an intense, focused beam 1 at the beam axis, and surrounding sidelobes which vary sinusoidally in intensity as a function of radial distance from the propagation axis. These sidelobes appear as a set of surrounding annular rings 2 of varying intensity. As shown in FIG. 2, the X beam also has an intense, focused beam 3 at the beam axis, but its surrounding sidelobes 4 taper off in intensity smoothly as a function of radial distance from the beam axis. A common characteristic of the sidelobes on both of these beam types is their symmetry throughout 360.degree. around the axis of propagation. Stated another way, the intensity of the sidelobes is independent of angular location around the propagation axis.
High sidelobes hinder the performance of limited diffraction beams. In ultrasonic medical imaging, for example, the echo make it more difficult to detect low scattering objects such as signals that result from the sidelobes reduce image contrast as small cysts. High sidelobes also increase the effective sampling volume, thus lowering the image resolution in tissue characterization.
High sidelobes are not unique to limited diffraction beams and much effort has been made in the past to reduce their deleterious effects on image quality. For example, Burckhardt et al. describe in an article "Focussing ultrasound over a large depth with an annular transducer--an alternative method", IEEE Trans. Sonics Ultrason., SU-22, No. 1, pp. 11-15, January 1975, cutting a ring transducer into eight or more equal segments and arranging the segments into two orthogonal groups. One group is used to transmit and the other to receive. The whole geometry is rotated to repeat the process and the resulting signals are added to the previously received signals. This method reduces the sidelobes of the ring to some extent. Macovski et al. describe in an article, "High-resolution B-scan systems using a circular array," Acoustic Holography, Vol. 6, N. Booth, Editor, pp. 121-143, 1975, weighting a ring transducer with the powers of the cosine and sine functions to transmit and receive, respectively. This results in sidelobe reductions in two orthogonal directions of the ring but leaves high sidelobes in other directions. Moshfeghi, describes in an article "Sidelobe suppression in annular array and axicon imaging systems," J. Acoust. Soc. Am., Vol. 83, No. 6, pp. 2202-2209, June, 1988, transmitting and receiving with different aperture sizes. However, this method does not reduce the sidelobes significantly.
A summation-subtraction method to reduce the sidelobes of the pulse-echo responses of limited diffraction ultrasonic beams has been developed by J-y. Lu and J. Greenleaf, "Sidelobe reduction for limited diffraction pulse-echo systems," IEEE Trans. Ultrason. Ferroelec. and Freq. Cont., Vol. 40, No. 6, pp. 735-746, November, 1993; "A study of sidelobe reduction for limited diffraction beams," IEEE 1993 Ultrason. Symp. Proc. 93CH3301-9, Vol. 2, pp. 1077-1082, 1993. This method has also been used successfully on other types of focused beams. The problem with it is that the final signals have a small dynamic range due to the subtraction of larger signals. Also, the method requires multiple transmissions that lower the image frame rate resulting in blurred images of moving objects such as the heart. Other methods for reducing the sidelobes of limited diffraction beams, such as deconvolution have been studied by J-y. Lu and J. Greenleaf, "Sidelobe reduction of nondiffracting pulse-echo images by deconvolution," Ultrason. Imag., Vol. 14, No. 2, p. 203, April 1992 (Abs), and dynamic focused reception have also been suggested. These are reviewed by J-y. Lu, H-h. Zou, and J. Greenleaf in an article, "Biomedical ultrasound beam forming," Ultrasound Med. Biol., Vol. 20, No. 5, pp. 403-428, July, 1994.